The longer history of Portugalâs presence in Africa is then considered, followed by sections on the age of the âscrambleâ for Africa, the 20th century, the (contested) economic aspect of Portugalâs experience in Africa, the liberation wars, and then the international response to Portugalâs colonial policies.
Regulatory Proteins. The binding of the myosin heads to the muscle actin is a highly-regulated process. When a muscle is in a resting state, actin and myosin are separated. To keep actin from binding to the active site on myosin, regulatory proteins block the molecular binding sites. Tropomyosin blocks myosin binding sites on actin molecules, preventing cross-bridge formation, which prevents contraction in a muscle without nervous input. The protein complex troponin binds to tropomyosin, helping to position it on the actin molecule.
Two of the important proteins are myosin, which forms the thick filament, and actin, which forms the thin filament. Myosin has a long, fibrous tail and a globular head, which binds to actin. The myosin head also binds to ATP, which is the source of energy for muscle movement.Myosin can only bind to actin when the binding sites on actin are exposed by calcium ions.Actin molecules are bound to the Z line, which forms the borders of the sarcomere.Other bands appear when the sarcomere is relaxed.he myosin head also binds to ATP, which is the source of energy for muscle movement. Myosin can only bind to actin when the binding sites on actin are exposed by calcium ions. Actin molecules are bound to the Z line, which forms the borders of the sarcomere. Other bands appear when the sarcomere is relaxed.